From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail172.messagelabs.com (mail172.messagelabs.com [216.82.254.3]) by kanga.kvack.org (Postfix) with ESMTP id 037056B00EE for ; Thu, 11 Aug 2011 07:14:38 -0400 (EDT) Date: Thu, 11 Aug 2011 13:14:23 +0200 From: Jan Kara Subject: Re: [PATCH 2/5] writeback: dirty position control Message-ID: <20110811111423.GD4755@quack.suse.cz> References: <20110806084447.388624428@intel.com> <20110806094526.733282037@intel.com> <1312811193.10488.33.camel@twins> <20110808141128.GA22080@localhost> <1312814501.10488.41.camel@twins> <20110808230535.GC7176@localhost> <1312910427.1083.68.camel@twins> <20110810223427.GA18227@quack.suse.cz> <20110811022952.GA11404@localhost> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20110811022952.GA11404@localhost> Sender: owner-linux-mm@kvack.org List-ID: To: Wu Fengguang Cc: Jan Kara , Peter Zijlstra , "linux-fsdevel@vger.kernel.org" , Andrew Morton , Christoph Hellwig , Dave Chinner , Greg Thelen , Minchan Kim , Vivek Goyal , Andrea Righi , linux-mm , LKML On Thu 11-08-11 10:29:52, Wu Fengguang wrote: > On Thu, Aug 11, 2011 at 06:34:27AM +0800, Jan Kara wrote: > > On Tue 09-08-11 19:20:27, Peter Zijlstra wrote: > > > On Tue, 2011-08-09 at 12:32 +0200, Peter Zijlstra wrote: > > > > > origin - dirty > > > > > pos_ratio = -------------- > > > > > origin - goal > > > > > > > > > which comes from the below [*] control line, so that when (dirty == goal), > > > > > pos_ratio == 1.0: > > > > > > > > OK, so basically you want a linear function for which: > > > > > > > > f(goal) = 1 and has a root somewhere > goal. > > > > > > > > (that one line is much more informative than all your graphs put > > > > together, one can start from there and derive your function) > > > > > > > > That does indeed get you the above function, now what does it mean? > > > > > > So going by: > > > > > > write_bw > > > ref_bw = dirty_ratelimit * pos_ratio * -------- > > > dirty_bw > > > > Actually, thinking about these formulas, why do we even bother with > > computing all these factors like write_bw, dirty_bw, pos_ratio, ... > > Couldn't we just have a feedback loop (probably similar to the one > > computing pos_ratio) which will maintain single value - ratelimit? When we > > are getting close to dirty limit, we will scale ratelimit down, when we > > will be getting significantly below dirty limit, we will scale the > > ratelimit up. Because looking at the formulas it seems to me that the net > > effect is the same - pos_ratio basically overrules everything... > > Good question. That is actually one of the early approaches I tried. > It somehow worked, however the resulted ratelimit is not only slow > responding, but also oscillating all the time. Yes, I think I vaguely remember that. > This is due to the imperfections > > 1) pos_ratio at best only provides a "direction" for adjusting the > ratelimit. There is only vague clues that if pos_ratio is small, > the errors in ratelimit should be small. > > 2) Due to time-lag, the assumptions in (1) about "direction" and > "error size" can be wrong. The ratelimit may already be > over-adjusted when the dirty pages take time to approach the > setpoint. The larger memory, the more time lag, the easier to > overshoot and oscillate. > > 3) dirty pages are constantly fluctuating around the setpoint, > so is pos_ratio. > > With (1) and (2), it's a control system very susceptible to disturbs. > With (3) we get constant disturbs. Well I had very hard time and > played dirty tricks (which you may never want to know ;-) trying to > tradeoff between response time and stableness.. Yes, I can see especially 2) is a problem. But I don't understand why your current formula would be that much different. As Peter decoded from your code, your current formula is: write_bw ref_bw = dirty_ratelimit * pos_ratio * -------- dirty_bw while previously it was essentially: ref_bw = dirty_ratelimit * pos_ratio So what is so magical about computing write_bw and dirty_bw separately? Is it because previously you did not use derivation of distance from the goal for updating pos_ratio? Because in your current formula write_bw/dirty_bw is a derivation of position... Honza -- Jan Kara SUSE Labs, CR -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: email@kvack.org